1. Field of the Invention
The present invention relates to a refining agent used for deoxidation, desulfurization, and dephosphorization of molten metal, especially molten steel, as well as methods for producing such a refining agent.
2. Description of the Prior Art
Recently, the so-called high purity-steels, which offer high reliability under very severe environmental conditions, have usually been produced by subjecting molten pig iron to desulfurization, dephosphorization, and other preliminary refining and then subsequently carrying out ladle refining of the molten steel or refining of the molten steel outside the steelmaking furnace. The ladle refining, etc. is meant to attain drastic desulfurization, removal of the Al.sub.2 O.sub.3 non-metallic inclusions, and modification of the non-metallic inclusions. The ladle refining, etc. is occasionally additionally meant to attain dephosphorization.
The refining agents differ according to the desired effect, but the general practice in ladle refining is to use both so-called flux, which is mainly composed of CaO, and a Ca-based metallic additive. The use of a Ca-based metallic additive using metallic Ca or a CaSi alloy is said to be indispensable for the modification of non-metallic inclusions.
The Ca-based metallic additives used are generally metallic Ca, a Ca--Si alloy, or a clad wire in which metallic Ca or both metallic Ca and Al are clad with an iron or aluminum sheath.
Metallic Ca and Ca--Si alloy are relatively inexpensive but are disadvantageous in that the metallic Ca or Ca--Si alloy exhibits a low reaction efficiency. The Ca--Si alloy can only be used for steels containing Si but cannot be used for Si-less Al-killed steels. In addition, fumes are generated due to addition of the Ca--Si alloy to the molten steel, which results in a poor working environment. On the other hand, while clad wire exhibits a high reaction efficiency, it is very expensive. Clade wire can therefore only be practically employed for refining certain kinds of steels.
The flux used is usually composed mainly of CaO and an additional one or more of Al.sub.2 O.sub.3 and CaF.sub.2. Use of flux together with the Ca-based metallic additive enables greater effectiveness in purification of the molten steel.
In conventional practice, the flux and the Ca-based metallic additive are added to the molten steel separately, e.g., the flux is added to melt first and then the Ca-based metallic additive is added. If the flux and Ca-based metallic additive are mixed with one another and simultaneously added to the molten steel, the metal phase and the flux phase substantially separate from one another in the molten steel, with the result that not only cannot the vaporization loss of the metallic Ca, which has a high vapor pressure at the temperature of molten steel, be suppressed, but also the non-metallic inclusions cannot be satisfactorily captured by the CaO. In conventional practice, however, since the flux and Ca-based metallic additive are added to the molten steel at a different time, a simultaneous and instantaneous reaction between the metallic Ca component and the flux cannot be realized.
Two of the more common Ca-based metallic additives used as refining agents comprise a Ca--Al alloy or a Ca--Si alloy. Several methods for producing a Ca--Al alloy have been previously proposed. In one method, CaO and Al.sub.2 O.sub.3 are reduced in an electric furnace by means of carbon. This method is difficult to carry out effectively on an industrial scale. In another method, briquettes are made of CaO and Al and are heated to a temperature of from 1500.degree. C. to 1600.degree. C. to cause a reaction between the CaO and Al and liquefaction of the Ca--Al alloy and slag. This method is, however, also difficult to carry out on an industrial scale because the reaction carried out at a high temperature in ambient air results in loss of Al and Ca due to vaporization, oxidation, and nitridation.
In addition, several methods for producing a Ca--Si alloy have been previously propsed. They include, for example, a method of reducing CaO by means of metallic Si, a method of reducing quartzite and quick lime by means of a carbonaceous reducing agent, and a method of reducing quartzite by means of calcium carbide (CaC.sub.2) and a carbonaceous reducing agent. In these methods, there is serious loss of the metallic Ca due to vaporization, and the Ca content of the resultant Ca--Si alloy is approximately 38% at the highest. Use of the products obtained by these methods as refining agents of metal would result in not only considerable consumption of the refining agent for treating the molten metal, but also the unavoidable formation of a SiO.sub.2 component in these products, which component would deleteriously combine with the CaO and thus impede desulfurization and dephosphorization. In order to remove the SiO.sub.2 component from the products and to eliminate the disadvantages, the slag must be necessarily separated from the alloy obtained by these methods.